Non-aqueous electrolyte solution for lithium secondary battery and lithium secondary battery including the same

ABSTRACT

The present invention relates to a non-aqueous electrolyte solution for a lithium secondary battery and a lithium secondary battery including the same, and particularly, to a non-aqueous electrolyte solution for a lithium secondary battery which includes a fluorine-containing compound capable of forming a stable film on the surface of an electrode as an additive, and a lithium secondary battery including the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.16/328,308 filed Feb. 26, 2019, a national phase entry under 35 U.S.C. §371 of International Application No. PCT/KR2018/001413 filed Feb. 1,2018, which claims priority from Korean Patent Application No.10-2017-0061201, filed on May 17, 2017, all of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to a non-aqueous electrolyte solutionadditive, and a non-aqueous electrolyte solution for a lithium secondarybattery and a lithium secondary battery which include the same, andparticularly, to a non-aqueous electrolyte solution for a lithiumsecondary battery and a lithium secondary battery which include anadditive capable of forming a stable film on the surface of anelectrode.

BACKGROUND ART

In line with miniaturization, lightweight, thin profile, and portabletrends in electronic devices according to the development of informationand telecommunications industry, the need for high energy densitybatteries used as power sources of such electronic devices hasincreased.

Lithium secondary batteries, as batteries that may best meet the need,have been adopted as power sources of many portable devices due to highenergy density and ease of design.

Recently, a lithium secondary battery, which may maintain excellentperformance not only at room temperature but also in a more severeoutside environment such as a high-temperature or low-temperatureenvironment, is required as the range of the lithium secondary batteriesused has expanded from conventional small electronic devices to largeelectronic devices, cars, or smart grids.

A lithium secondary battery is composed of a carbon material negativeelectrode capable of intercalating and deintercalating lithium ions, apositive electrode formed of a lithium-containing transition metaloxide, and a non-aqueous electrolyte solution, wherein charge anddischarge may be possible, because lithium ions, which are dischargedfrom a positive electrode active material by first charging, may play arole in transferring energy while moving between both electrodes, forexample, the lithium ions are intercalated into a negative electrodeactive material, for example, carbon particles, and deintercalatedduring discharging.

However, performance degradation of the positive electrode occurs whilethe positive electrode active material is structurally collapsed as thecharge and discharge proceed. Also, metal ions eluted from the surfaceof the positive electrode during the collapse of the positive electrodestructure may degrade the negative electrode while the metal ions areelectrodeposited on the negative electrode. Such a battery performancedegradation phenomenon tends to be more accelerated when the potentialof the positive electrode is increased or the battery is exposed to hightemperature.

Thus, there is a need to develop a lithium secondary battery having anew configuration which may address the above limitations.

PRIOR ART DOCUMENT Patent Documents

-   (Patent Document 1) Japanese Patent No. 4940625-   (Patent Document 2) Chinese Patent Application Laid-open Publication    No. 103326068

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides a non-aqueous electrolytesolution for a lithium secondary battery which includes a non-aqueouselectrolyte solution additive capable of forming a stable film on thesurface of an electrode.

Another aspect of the present invention provides a lithium secondarybattery in which overall performance, such as cycle lifecharacteristics, are improved by including the non-aqueous electrolytesolution for a lithium secondary battery.

Technical Solution

According to an aspect of the present invention, there is provided anon-aqueous electrolyte solution for a lithium secondary batteryincluding:

a lithium salt,

an organic solvent, and

a non-aqueous electrolyte solution additive represented by Formula 1.

In Formula 1,

X is a linear or nonlinear alkyl group having 1 to 5 carbon atoms or—SiR₂R₃R₄, wherein R₂ to R₄ are each independently an alkyl group having1 to 5 carbon atoms, and

R₁ is an alkylene group having 2 to 3 carbon atoms which is substitutedwith at least one fluorine atom, or an alkylene group having 2 to 3carbon atoms which is substituted with an alkyl group having 1 to 3carbon atoms that includes at least one fluorine atom.

Specifically, X is —SiR₂R₃R₄, wherein R₂ to R₄ are each independently analkyl group having 1 to 5 carbon atoms, and R₁ is an alkylene grouphaving 2 carbon atoms which is substituted with at least one fluorineatom, or an alkylene group having 2 carbon atoms which is substitutedwith an alkyl group having 1 to 3 carbon atoms that includes at leastone fluorine atom.

The non-aqueous electrolyte solution additive may be included in anamount of 0.1 wt % to 10 wt %, for example, 0.5 wt % to 5 wt % based ona total weight of the non-aqueous electrolyte solution.

According to another aspect of the present invention,

there is provided a lithium secondary battery including a negativeelectrode, a positive electrode, a separator disposed between thenegative electrode and the positive electrode, and the non-aqueouselectrolyte solution of the present invention.

The negative electrode may include a single material selected from thegroup consisting of a carbon-based active material, a silicon-basedactive material, a metal-based active material alloyable with lithium,and a lithium-containing nitride, or a mixture of two or more thereof.

Advantageous Effects

According to an embodiment of the present invention, in a case in whicha fluorine-containing compound is included in a non-aqueous electrolytesolution, since an inorganic component, such as LiF, is increased in thenon-aqueous electrolyte solution, a more stable ionic conductive filmmay be formed on surfaces of a negative electrode and a positiveelectrode. Furthermore, in a case in which a negative electrodeincluding a silicon-based negative electrode active material as well asthe non-aqueous electrolyte solution including the fluorine-containingcompound is used, a lithium secondary battery having more improvedoverall performance, such as cycle life characteristics, may beprepared.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail.

It will be understood that words or terms used in the specification andclaims should be interpreted as having a meaning that is consistent withtheir meaning in the context of the relevant art and the technical ideaof the invention, based on the principle that an inventor may properlydefine the meaning of the words or terms to best explain the invention.

In a lithium secondary battery among electrochemical devices, a kind ofpassivation layer is formed at a positive electrode of the battery,particularly, at a position where surface bonding exists or at anactivated position by an electrochemical oxidation decompositionreaction, wherein the passivation layer increases impedance when lithiumions are co-intercalated into a positive electrode active material.Also, while an excessive amount of lithium ions is discharged from thepositive electrode during overcharge or high-temperature storage,structural collapse of the positive electrode active material or achemical dissolution reaction by an electrolyte solution occurs so thations, such as cobalt (Co), manganese (Mn), and nickel (Ni), are elutedfrom the positive electrode active material. These reactions not onlylead to performance degradation of the positive electrode itself, butalso cause the collapse of the positive structure as well as anelectrolyte solution side reaction to degrade overall performance of thesecondary battery.

The present invention aims at providing a non-aqueous electrolytesolution including a fluorine-containing compound as an additive, and,furthermore, aims at providing a lithium secondary battery, in whichoverall performance, such as cycle life characteristics, of the batteryare improved by forming a stable film on surfaces of positive electrodeand negative electrode, by including the non-aqueous electrolytesolution.

Specifically, in an embodiment of the present invention, provided is anon-aqueous electrolyte solution for a lithium secondary batteryincluding:

a lithium salt,

an organic solvent, and

a non-aqueous electrolyte solution additive represented by Formula 1.

In Formula 1,

X is a linear or nonlinear alkyl group having 1 to 5 carbon atoms or—SiR₂R₃R₄, wherein R₂ to R₄ are each independently an alkyl group having1 to 5 carbon atoms, and

R₁ is an alkylene group having 2 to 3 carbon atoms which is substitutedwith at least one fluorine atom, or an alkylene group having 2 to 3carbon atoms which is substituted with an alkyl group having 1 to 3carbon atoms that includes at least one fluorine atom.

Specifically, in Formula 1, X is —SiR₂R₃R₄, wherein R₂ to R₄ are eachindependently an alkyl group having 1 to 5 carbon atoms, and R₁ is analkylene group having 2 carbon atoms which is substituted with at leastone fluorine atom, or an alkylene group having 2 carbon atoms which issubstituted with an alkyl group having 1 to 3 carbon atoms that includesat least one fluorine atom.

Specific examples of the compound represented by Formula 1 may be atleast one compound selected from the group consisting of compoundsrepresented by Formulae 1a to 1h below.

In a case in which the compound containing at least one fluorine atom isincluded as the non-aqueous electrolyte solution additive with thecompound represented by Formula 1 in the structure, an amount of aninorganic component, such as LiF, is increased in the non-aqueouselectrolyte solution. As a result, fluorine (F) improves a bondingeffect, for example, agglomeration of organic components for forming anSEI to each other or a good attachment of the organic components forforming an SEI to a surface of an active material, by a reaction inwhich the fluorine is coordination bonded with lithium (Li) ions of theorganic SEI component, and thus, a more stable ionic conductive film maybe formed on the surfaces of the negative electrode and the positiveelectrode.

In particular, with respect to the compounds having a Si—O bond amongthe compounds represented by Formulae 1a to 1h, it is advantageous inthat they may also play a role as a HF scavenger. Furthermore, 5-cyclicmaterials may achieve slightly better overall performance than 6-cyclicmaterials due to structural stability of a ring.

With respect to fluoroethylene carbonate (FEC) that is typically knownas an additive for improving the amount of the LIF component, it isknown that a large amount of gas may be generated because the FEC isdecomposed in the electrolyte solution during high-temperature storage.In contrast, with respect to the compound having a phosphate structureof the present invention, gas generation is not only low, but it mayalso have an effect of stabilizing an anion of the lithium salt, forexample, PF₆ ⁻. Thus, the compound having a phosphate structure of thepresent invention may further improve life and high-temperature storagecharacteristics in comparison to a carbonate-based compound such as FEC.

In a case in which the non-aqueous electrolyte solution additive isincluded in an amount of about 0.1 wt % to about 10 wt %, for example,0.5 wt % to 5 wt % based on a total weight of the non-aqueouselectrolyte solution, a secondary battery having more improved overallperformance may be prepared. Specifically, in a case in which the amountof the additive is 0.1 wt % or more, a better film-forming effect may beobtained, and, in a case in which the amount of the additive is 10 wt %or less, a decrease in capacity of the battery due to a side reaction ofthe surplus of the non-aqueous electrolyte solution additive, anincrease in viscosity of the electrolyte solution, the resultingincrease in resistance, and an ionic conductivity reduction phenomenonmay be prevented.

In an embodiment, any lithium salt typically used in an electrolytesolution for a lithium secondary battery may be used as the lithium saltincluded in the non-aqueous electrolyte solution of the presentinvention without limitation, and, for example, the lithium salt mayinclude Li⁺ as a cation, and may include at least one selected from thegroup consisting of F, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻,AlO₄ ⁻, AlCl₄ ⁻, PF₆ ⁻, SbFe⁻, AsF₆ ⁻, BF₂C₂O₄ ⁻, BC₄O₈ ⁻, (CF₃)₂PF₄ ⁻,(CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, C₄F₉SO₃ ⁻,CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻,CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and (CF₃CF₂SO₂)₂N⁻ as ananion. One or, if necessary, a mixture of two or more thereof may beused as the lithium salt. The lithium salt may be appropriately changedin a normally usable range, but may be included in a concentration of0.8 M to 2.0 M in the electrolyte solution in order to obtain an optimumanti-corrosion film-forming effect on the surface of the electrode.

Also, any organic solvent typically used in an electrolyte solution fora lithium secondary battery may be used as the organic solvent includedin the non-aqueous electrolyte solution of the present invention withoutlimitation, and, for example, an ether-based solvent, an ester-basedsolvent, or an amide-based solvent may be used alone or as a mixture oftwo or more thereof.

As the ether-based solvent among the organic solvents, any one selectedfrom the group consisting of dimethyl ether, diethyl ether, dipropylether, methylethyl ether, methylpropyl ether, and ethylpropyl ether, ora mixture of two or more thereof may be used, but the present inventionis not limited thereto.

Furthermore, the ester-based solvent may include at least one compoundselected from the group consisting of a cyclic carbonate compound, alinear carbonate compound, a linear ester compound, and a cyclic estercompound.

Among these compounds, specific examples of the cyclic carbonatecompound may be any one selected from the group consisting of ethylenecarbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate,2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylenecarbonate, vinylene carbonate, and fluoroethylene carbonate (FEC), or amixture of two or more thereof.

Also, specific examples of the linear carbonate compound may be any oneselected from the group consisting of dimethyl carbonate (DMC), diethylcarbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC),methylpropyl carbonate, and ethylpropyl carbonate, or a mixture of twoor more thereof, but the present invention is not limited thereto.

Specific examples of the linear ester compound may be any one selectedfrom the group consisting of methyl acetate, ethyl acetate, propylacetate, methyl propionate, ethyl propionate, propyl propionate, andbutyl propionate, or a mixture of two or more thereof, but the presentinvention is not limited thereto.

Specific examples of the cyclic ester compound may be any one selectedfrom the group consisting of γ-butyrolactone, γ-valerolactone,γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of twoor more thereof, but the present invention is not limited thereto.

In this case, among the ester-based solvents, since the cycliccarbonate-based compound well dissociates the lithium salt in theelectrolyte due to high permittivity as a highly viscous organicsolvent, the cyclic carbonate-based compound may be preferably used.Since an electrolyte solution having high electrical conductivity may beprepared when the above cyclic carbonate-based compound is mixed withthe low viscosity, low permittivity linear carbonate-based compound,such as dimethyl carbonate and diethyl carbonate, in an appropriateratio, the cyclic carbonate-based compound may be more preferably used.

The non-aqueous electrolyte solution of the present invention mayfurther include an additive for forming a solid electrolyte interface(SEI), if necessary. As the additive for forming an SEI which may beused in the present invention, at least one of vinylene carbonate,fluoroethylene carbonate, vinyl ethylene carbonate, cyclic sulfite,saturated sultone, unsaturated sultone, a non-cyclic sulfone, analkylsilyl compound, and an inorganic additive may be mixed and used.

In a case in which the additive for forming an SEI is included, thefilm-forming effect may be further improved when the additive forforming an SEI is included in an amount of about 0.01 wt % or more basedon a total amount of the non-aqueous electrolyte solution.

Among the additives for forming an SEI, typical examples of the cyclicsulfite may be ethylene sulfite, methyl ethylene sulfite, ethyl ethylenesulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite,propylene sulfite, 4,5-dimethyl propylene sulfite, 4,5-diethyl propylenesulfite, 4,6-dimethyl propylene sulfite, 4,6-diethyl propylene sulfite,or 1,3-butylene glycol sulfite.

Among the additives for forming an SEI, typical examples of thesaturated sultone may be 1,3-propane sultone and 1,4-butane sultone, andthe unsaturated sultone may include ethene sultone, 1,3-propene sultone,1,4-butene sultone, or 1-methyl-1,3-propene sultone.

Among the additives for forming an SEI, typical examples of thenon-cyclic sulfone may be divinyl sulfone, dimethyl sulfone, diethylsulfone, methyl ethyl sulfone, or methyl vinyl sulfone.

Among the additives for forming an SEI, typical examples of thealkylsilyl compound may be tris(trimethylsilyl) phosphate,tris(trimethylsilyl) phosphite, tris(triethylsilyl) phosphate,tris(triethylsilyl) phosphite, tris(trimethylsilyl) borate, ortris(triethylsilyl) borate.

Among the additives for forming an SEI, typical examples of theinorganic additive may be lithium tetrafluoroborate, lithiumdifluoro(bisoxalate) phosphate, lithium difluoro phosphate, lithiumtetrafluorooxalato phosphate, lithium bis(fluorosulfonyl)imide, lithiumbis(trifluoromethyl sulfonyl)imide, lithium oxalyldifluoroborate, orlithium bis(oxalato)borate.

Also, in an embodiment of the present invention,

provided is a lithium secondary battery including a positive electrode,a negative electrode, a separator disposed between the positiveelectrode and the negative electrode, and a non-aqueous electrolytesolution, wherein the non-aqueous electrolyte solution includes thenon-aqueous electrolyte solution of the present invention.

Specifically, the lithium secondary battery of the present invention maybe prepared by injecting the non-aqueous electrolyte solution of thepresent invention into an electrode assembly composed of the positiveelectrode, the negative electrode, and the separator disposed betweenthe positive electrode and the negative electrode. In this case, anypositive electrode, negative electrode, and separator, which havetypically been used in the preparation of a lithium secondary battery,may be used as the positive electrode, negative electrode, and separatorwhich constitute the electrode assembly.

In this case, the positive electrode may be prepared by forming apositive electrode material mixture layer on a positive electrodecollector.

The positive electrode material mixture layer may be formed by coatingthe positive electrode collector with a positive electrode slurryincluding a positive electrode active material, a binder, a conductiveagent, and a solvent, and then drying and rolling the coated positiveelectrode collector.

The positive electrode collector is not particularly limited so long asit has conductivity without causing adverse chemical changes in thebattery, and, for example, stainless steel, aluminum, nickel, titanium,fired carbon, or aluminum or stainless steel that is surface-treatedwith one of carbon, nickel, titanium, silver, or the like may be used.

The positive electrode active material is a compound capable ofreversibly intercalating and deintercalating lithium, wherein thepositive electrode active material may specifically include a lithiumcomposite metal oxide including lithium and at least one metal such ascobalt, manganese, nickel, or aluminum. Specifically, the lithiumcomposite metal oxide may include lithium-manganese-based oxide (e.g.,LiMnO₂, LiMn₂O₄, etc.), lithium-cobalt-based oxide (e.g., LiCoO₂, etc.),lithium-nickel-based oxide (e.g., LiNiO₂, etc.),lithium-nickel-manganese-based oxide (e.g., LiNi_(1-Y)Mn_(Y)O₂ (where0<Y<1), LiMn_(2-Z)Ni_(Z)O₄ (where 0<Z<2), etc.),lithium-nickel-cobalt-based oxide (e.g., LiNi_(1-Y1)Co_(Y1)O₂ (where0<Y1<1), lithium-manganese-cobalt-based oxide (e.g.,LiCo_(1-Y2)Mn_(Y2)O₂ (where 0<Y2<1), LiMn_(2-Z1)Co_(z1)O₄ (where0<Z1<2), etc.), lithium-nickel-manganese-cobalt-based oxide (e.g.,Li(Ni_(p)Co_(q)Mn_(r1))O₂ (where 0<p<1, 0<q<1, 0<r1<1, and p+q+r1=1) orLi(Ni_(p1)Co_(q1)Mn_(r2))O₄ (where 0<p1<2, 0<q1<2, 0<r2<2, andp1+q1+r2=2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide(e.g., Li(Ni_(p2)Co_(q2)Mn_(r3)M_(S2))O₂ (where M is selected from thegroup consisting of aluminum (Al), iron (Fe), vanadium (V), chromium(Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo),and p2, q2, r3, and s2 are atomic fractions of each independentelements, wherein 0<p2<1, 0<q2<1, 0<r3<1, 0<S2<1, and p2+q2+r3+S2=1),etc.), and any one thereof or a compound of two or more thereof may beincluded. Among these materials, in terms of the improvement of capacitycharacteristics and stability of the battery, the lithium compositemetal oxide may include LiCoO₂, LiMnO₂, LiNiO₂, lithium nickel manganesecobalt oxide (e.g., Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂,Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, or Li(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂), orlithium nickel cobalt aluminum oxide (e.g.,LiNi_(0.8)Co_(0.5)Al_(0.05)O₂, etc.), and, in consideration of asignificant improvement due to the control of type and content ratio ofelements constituting the lithium composite metal oxide, the lithiumcomposite metal oxide may include Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂,Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, Li(Ni_(0.7)Mn_(0.15)Co_(0.15))O₂, orLi(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂, and any one thereof or a mixture of twoor more thereof may be used.

The positive electrode active material may be included in an amount of80 wt % to 99 wt % based on a total weight of solid content in thepositive electrode slurry.

The conductive agent is commonly added in an amount of 1 wt % to 30 wt %based on the total weight of the solid content in the positive electrodeslurry.

Any conductive agent may be used without particular limitation so longas it has suitable conductivity without causing adverse chemical changesin the battery, and, for example, a conductive material such as:graphite; a carbon-based material such as carbon black, acetylene black,Ketjen black, channel black, furnace black, lamp black, and thermalblack; conductive fibers such as carbon fibers or metal fibers; metalpowder such as fluorocarbon powder, aluminum powder, and nickel powder;conductive whiskers such as zinc oxide whiskers and potassium titanatewhiskers; conductive metal oxide such as titanium oxide; orpolyphenylene derivatives may be used. Specific examples of a commercialconductive agent may be acetylene black-based products (Chevron ChemicalCompany, Denka black (Denka Singapore Private Limited), or Gulf OilCompany), Ketjen black, ethylene carbonate (EC)-based products (ArmakCompany), Vulcan XC-72 (Cabot Company), and Super P (Timcal Graphite &Carbon).

The binder is a component that assists in the binding between the activematerial and the conductive agent and in the binding with the currentcollector, wherein the binder is commonly added in an amount of 1 wt %to 30 wt % based on the total weight of the solid content in thepositive electrode slurry. Examples of the binder may be polyvinylidenefluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC),starch, hydroxypropylcellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,an ethylene-propylene-diene terpolymer (EPDM), a sulfonated EPDM, astyrene-butadiene rubber, a fluoro rubber, various copolymers, and thelike.

The solvent may include an organic solvent, such asN-methyl-2-pyrrolidone (NMP), and may be used in an amount such thatdesirable viscosity is obtained when the positive electrode activematerial as well as selectively the binder and the conductive agent areincluded. For example, the solvent may be included in an amount suchthat a concentration of the solid content in the slurry including thepositive electrode active material as well as selectively the binder andthe conductive agent is in a range of 50 wt % to 95 wt %, for example,70 wt % to 90 wt %.

Also, the negative electrode may be prepared by forming a negativeelectrode material mixture layer on a negative electrode collector.

The negative electrode material mixture layer may be formed by coatingthe negative electrode collector with a slurry including a negativeelectrode active material, a binder, a conductive agent, and a solvent,and then drying and rolling the coated negative electrode collector.

The negative electrode collector generally has a thickness of 3 μm to500 μm. The negative electrode collector is not particularly limited solong as it has high conductivity without causing adverse chemicalchanges in the battery, and, for example, copper, stainless steel,aluminum, nickel, titanium, fired carbon, copper or stainless steel thatis surface-treated with one of carbon, nickel, titanium, silver, or thelike, an aluminum-cadmium alloy, or the like may be used. Also, similarto the positive electrode collector, the negative electrode collectormay have fine surface roughness to improve bonding strength with thenegative electrode active material, and the negative electrode collectormay be used in various shapes such as a film, a sheet, a foil, a net, aporous body, a foam body, a non-woven fabric body, and the like.

A compound capable of reversibly intercalating and deintercalatinglithium may be used as the negative electrode active material. Specificexamples of the negative electrode active material may include a singlematerial selected from the group consisting of a carbon-based activematerial, a silicon-based active material, a metal-based active materialalloyable with lithium, and a lithium-containing nitride, or a mixtureof two or more thereof.

Typical examples of the carbon-based active material may include naturalgraphite, artificial graphite, expanded graphite, carbon fibers,non-graphitizable, carbon, carbon black, carbon nanotubes, fullerene, oractivated carbon, and the carbon-based active material may be usedwithout limitation as long as it is conventionally used in a carbonmaterial for a lithium secondary battery.

Typical examples of the silicon-based active material may include atleast one selected from the group consisting of silicon, an alloy withsilicon, SiB₄, SiB₆, Mg₂Si, Ni₂Si, TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂,CrSi₂, Cu₅Si, FeSi₂, MnSi₂, NbSi₂, TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄,Si₂N₂O, SiO_(v) (0.5≤v≤1.2), and LiSiO.

Typical examples of the metal-based active material may includecompounds containing at least one metallic element selected from thegroup consisting of Al, tin (Sn), silver (Ag), bismuth (Bi), Mg, zinc(Zn), indium (In), germanium (Ge), lead (Pb), palladium (Pd), platinum(Pt), Ti, antimony (Sb), gallium (Ga), Mn, Fe, Co, Ni, copper (Cu),strontium (Sr), and barium (Ba). The metallic compounds may be used inany form, such as a simple substance, an alloy, an oxide (TiO₂, SnO₂,etc), a nitride, a sulfide, a boride, or an alloy with lithium, but thesimple substance, alloy, oxide, and alloy with lithium may have highcapacity.

In a case in which a negative electrode including the silicon-basedactive material or both of the silicon-based active material and thecarbon-based active material, as the negative electrode active material,is used, a lithium secondary battery having improved overallperformance, such as cycle life characteristics, may be prepared. Thatis, with respect to the silicon-based active material, since a change involume is very large during charge and discharge, stability of the SEIis significantly reduced. In contrast, in a case in which thenon-aqueous electrolyte solution including the additive of the presentinvention is used, since a sufficient SEI capable of compensating forthe shortcomings of the silicon-based active material may be formed asthe amount of LiF in the non-aqueous electrolyte solution is increased,an effect of improving lifetime of the secondary battery using thesilicon-based active material is more significant.

The negative electrode active material may be included in an amount of80 wt % to 99 wt % based on a total weight of solid content in thenegative electrode slurry.

The binder is a component that assists in the binding between theconductive agent, the active material, and the current collector,wherein the binder is commonly added in an amount of 1 wt % to 30 wt %based on the total weight of the solid content in the negative electrodeslurry. Examples of the binder may be polyvinylidene fluoride (PVDF),polyvinyl alcohol, carboxymethylcellulose (CMC), starch,hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone,tetrafluoroethylene, polyethylene, polypropylene, anethylene-propylene-diene polymer (EPDM), a sulfonated EPDM, astyrene-butadiene rubber, a fluoro rubber, and various copolymersthereof.

The conductive agent is a component for further improving theconductivity of the negative electrode active material, wherein theconductive agent may be added in an amount of 1 wt % to 20 wt % based onthe total weight of the solid content in the negative electrode slurry.Any conductive agent may be used without particular limitation so longas it has conductivity without causing adverse chemical changes in thebattery, and, for example, a conductive material, such as: graphite suchas natural graphite or artificial graphite; carbon black such asacetylene black, Ketjen black, channel black, furnace black, lamp black,and thermal black; conductive fibers such as carbon fibers and metalfibers; metal powder such as fluorocarbon powder, aluminum powder, andnickel powder; conductive whiskers such as zinc oxide whiskers andpotassium titanate whiskers; conductive metal oxide such as titaniumoxide; or polyphenylene derivatives, may be used.

The solvent may include water or an organic solvent, such as NMP andalcohol, and may be used in an amount such that desirable viscosity isobtained when the negative electrode active material as well asselectively the binder and the conductive agent are included. Forexample, the solvent may be included in an amount such that aconcentration of the solid content in the slurry including the negativeelectrode active material as well as selectively the binder and theconductive agent is in a range of 50 wt % to 95 wt %, for example, 70 wt% to 90 wt %.

Also, a typical porous polymer film conventionally used as a separator,for example, a porous polymer film prepared from a polyolefin-basedpolymer, such as an ethylene homopolymer, a propylene homopolymer, anethylene/butene copolymer, an ethylene/hexene copolymer, and anethylene/methacrylate copolymer, may be used alone or in a laminationtherewith as the separator. Also, a typical porous nonwoven fabric, forexample, a nonwoven fabric formed of high melting point glass fibers orpolyethylene terephthalate fibers may be used, but the present inventionis not limited thereto.

A shape of the lithium secondary battery of the present invention is notparticularly limited, but a cylindrical type using a can, a prismatictype, a pouch type, or a coin type may be used.

Hereinafter, the present invention will be described in more detailaccording to examples. However, the invention may be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein. Rather, these example embodiments areprovided so that this description will be thorough and complete, andwill fully convey the scope of the present invention to those skilled inthe art.

EXAMPLES Example 1

(Non-Aqueous Electrolyte Solution Preparation)

After ethylene carbonate (EC) and ethyl methyl carbonate (EMC) weremixed in a ratio of 30:70 (vol %), an organic mixed solution wasprepared by dissolving LiPF₆ to a concentration of 1 M.

The organic mixed solution and the additive of the present inventionwere added as listed in the following Table 1 to prepare a non-aqueouselectrolyte solution.

(Positive Electrode Preparation)

40 parts by weight of a solid, in which a ternary active material(Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂), carbon black as a conductive agent,and polyvinylidene fluoride (PVDF) as a binder, were mixed in a ratio of90:5:5 (wt %), was added to 100 parts by weight ofN-methyl-2-pyrrolidone (NMP) to prepare a positive electrode activematerial slurry. A 100 μm thick positive electrode collector (Al thinfilm) was coated with the positive electrode active material slurry,dried, and roll-pressed to prepare a positive electrode.

(Negative Electrode Preparation)

100 parts by weight of a solid, in which natural graphite and SiO_(x)(0<x<1), as a negative electrode active material, PVDF as a binder, andcarbon black as a conductive agent, were mixed in a ratio of 90:5:2:3(wt %), was added to 100 parts by weight of NMP to prepare a negativeelectrode active material slurry. A 90 μm thick negative electrodecollector (Cu thin film) was coated with the negative electrode activematerial slurry, dried, and roll-pressed to prepare a negativeelectrode.

(Secondary Battery Preparation)

After an electrode assembly was prepared by stacking a polyethyleneporous film with the positive electrode and negative electrode preparedby the above-described methods, the electrode assembly was put in apouch-type battery case, the non-aqueous electrolyte solution thusprepared was injected thereinto, and the battery case was sealed toprepare a lithium secondary battery.

Examples 2 to 13

Electrolyte solutions and secondary batteries including the same wererespectively prepared in the same manner as in Example 1 except thatadditives were respectively included in amounts listed in the followingTable 1 during the preparation of the non-aqueous electrolyte solutionin Example 1.

Comparative Example 1

A non-aqueous electrolyte solution and a secondary battery including thesame were prepared in the same manner as in Example 1 except that anadditive was not included during the preparation of the non-aqueouselectrolyte solution in Example 1.

Comparative Example 2

A non-aqueous electrolyte solution and a secondary battery including thesame were prepared in the same manner as in Example 1 except that 0.5 gof a compound of the following Formula 2, instead of the compound ofFormula 1a, was included during the preparation of the non-aqueouselectrolyte solution in Example 1.

Comparative Example 3

A non-aqueous electrolyte solution and a secondary battery including thesame were prepared in the same manner as in Example 1 except that 3 g offluoroethylene carbonate, instead of the compound of Formula 1a, wasincluded during the preparation of the non-aqueous electrolyte solutionin Example 1.

EXPERIMENTAL EXAMPLES Experimental Example 1

Each of the secondary batteries prepared in Examples 1 to 13 andComparative Examples 1 to 3 was charged at a 0.8 C rate to 4.35 V undera constant current/constant voltage condition, cut-off charged at 0.05C, and discharged at 0.5 C to a voltage of 3.0 V (initial dischargecapacity). Subsequently, after each of the secondary batteries wascharged at a 0.8 C rate to 4.35 V under a constant current/constantvoltage condition, cut-off charged at 0.05 C, and stored at 60° C. for 2weeks, a thickness increase rate was confirmed by measuring a thicknessof the pouch type battery. Thereafter, each battery was discharged at0.5 C to a voltage of 3.0 V at room temperature and discharge capacitywas measured (residual discharge capacity). Again, each battery wascharged at a 0.8 C rate to 4.35 V under a constant current/constantvoltage condition, cut-off charged at 0.05 C, and discharged at 0.5 C toa voltage of 3.0 V to measure discharge capacity (recovery dischargecapacity).

The thickness increase rate was expressed as a percentage (%) relativeto an initial thickness, the residual discharge capacity and therecovery discharge capacity were expressed as a percentage (%) relativeto the initial discharge capacity, and the results thereof are presentedin the following Table 1.

Experimental Example 2

Each of the secondary batteries prepared in Examples 1 to 13 andComparative Examples 1 to 3 was charged at a 0.8 C rate to 4.35 V undera constant current/constant voltage condition, cut-off charged at 0.05C, and discharged at 0.5 C to a voltage of 3.0 V. Subsequently, chargingat a 0.8 C rate to 4.35 V under a constant current/constant voltagecondition, cut-off charging at 0.05 C, and discharging at 0.5 C to avoltage of 3.0 V at room temperature were set as one cycle, and cyclecapacity retention after 200 cycles was expressed as a percentage (%)relative to first cycle capacity and listed in Table 1 below.

TABLE 1 Organic Residual Recovery Cycle mixed Additive Thicknessdischarge discharge capacity solution Amount increase capacity capacityretention Examples amount (g) Formula (g) rate (%) (%) (%) (%) Example 199.5 1a 0.5 105 91 97 83 Example 2 99.5 1b 0.5 105 91 96 81 Example 399.5 1c 0.5 104 94 98 85 Example 4 99.5 1d 0.5 107 92 97 84 Example 599.5 1e 0.5 105 95 98 87 Example 6 99.5 1f 0.5 106 89 94 78 Example 799.5 1g 0.5 103 91 96 81 Example 8 99.5 1h 0.5 104 89 95 84 Example 9 971a 3   109 93 98 87 Example 10 90 1a 10   112 94 96 80 Example 11 99.951a  0.05 118 84 90 68 Example 12 87 1a 13   125 88 92 47 Example 13 99.01a/VC 0.5/0.5 108 93 99 87 Comparative 100 X X 121 82 88 65 Example 1Comparative 99.5 2 0.5 110 86 90 73 Example 2 Comparative 97 FEC 3   13688 92 79 Example 3

As illustrated in Table 1, with respect to the secondary batteries ofExamples 1 to 11 which respectively included the non-aqueous electrolytesolutions including the compound of the present invention as anadditive, it may be understood that battery thickness increase ratesduring high-temperature storage, residual discharge capacities, recoverydischarge capacities, and cycle capacity retentions were better thanthose of the secondary battery of Comparative Example 1 which includedthe non-aqueous electrolyte solution in which the additive was not used.

Also, with respect to the compounds having a Si—O bond among thecompounds represented by Formulae 1a to 1h, it was advantageous in thatthey may also play a role as a HF scavenger. Thus, with respect to thesecondary batteries of Examples 3, 5, and 7 which respectively includedthe non-aqueous electrolyte solutions respectively including thecompounds represented by Formulae 1c, 1e, and 1g which had the Si—Obond, it may be understood that the battery thickness increase ratesduring high-temperature storage, residual discharge capacities, recoverydischarge capacities, and cycle capacity retentions were better thanthose of the secondary batteries of Example 2, 4, and 6 respectivelyincluding the compounds represented by Formulae 1b, 1d, and 1f which didnot have the Si—O bond.

Furthermore, with respect to the secondary batteries of Examples 1 to 11which respectively included the non-aqueous electrolyte solutionsincluding the compound of the present invention as an additive, sincegas generation was reduced in comparison to the secondary battery ofComparative Example 3 which included the non-aqueous electrolytesolution using FEC as an additive, it may be understood that the batterythickness increase rates during high-temperature storage weresignificantly reduced.

Also, it may be understood that the secondary batteries of Examples 1 to11 of the present invention had better battery thickness increase ratesduring high-temperature storage, residual discharge capacities, recoverydischarge capacities, and cycle capacity retentions than the secondarybattery of Comparative Example 2 which included the non-aqueouselectrolyte solution using the phosphate-based compound of Formula 2without fluorine as an additive.

The invention claimed is:
 1. A lithium secondary battery comprising anegative electrode, a positive electrode, a separator disposed betweenthe negative electrode and the positive electrode, and a non-aqueouselectrolyte solution, wherein the negative electrode comprises asilicon-based active material; and wherein the non-aqueous electrolytesolution comprises a lithium salt, an organic solvent, and a firstadditive represented by Formula 1:

wherein, in Formula 1, X is a linear or nonlinear unsubstituted alkylgroup having 1 to 5 carbon atoms, and R₁ is an alkylene group having 2to 3 carbon atoms which is substituted with at least one fluorine atom,or an alkylene group having 2 to 3 carbon atoms which is substitutedwith an alkyl group having 1 to 3 carbon atoms that includes at leastone fluorine atom, provided when R₁ is an alkylene group having 2 carbonatoms, each of the 2 carbon atoms is substituted with no more than onefluorine atom.
 2. The lithium secondary battery of claim 1, wherein thesilicon-based active material is at least one selected from the groupconsisting of silicon, an alloy with silicon, SiB₄, SiB₆, Mg₂Si, Ni₂Si,TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₅Si, FeSi₂, MnSi₂, NbSi₂,TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiO_(v) (0.5≤v≤1.2), andLiSiO.
 3. The lithium secondary battery of claim 1, wherein the positiveelectrode comprises a lithium composite metal oxide.
 4. The lithiumsecondary battery of claim 3, wherein the lithium composite metal oxidecomprises LiCoO₂, LiMnO₂, LiNiO₂, lithium nickel manganese cobalt oxide,or lithium nickel cobalt aluminum oxide.
 5. The lithium secondarybattery of claim 4, wherein the lithium nickel manganese cobalt oxidecomprises Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂,Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, Li(Ni_(0.7)Mn_(0.5)Co_(0.15))O₂,Li(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂, or a mixture of two or more thereof. 6.The lithium secondary battery of claim 1, the lithium salt comprisesLi⁺, as a cation, and at least one of F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻,BF₄ ⁻, ClO₄ ⁻, AlO₄ ⁻, AlCl₄ ⁻, PF₆ ⁻, SbF₆ ⁻, AsF₆ ⁻, BF₂C₂O₄ ⁻, BC₄O₈⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻,C₄F₉SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻,(CF₃SO₂)₂CH⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, or(CF₃CF₂SO₂)₂N⁻ as an anion.
 7. The lithium secondary battery of claim 1,wherein the lithium salt is included in a concentration of 0.8 M to 2.0M in the non-aqueous electrolyte solution.
 8. The lithium secondarybattery of claim 1, wherein the organic solvent comprises one selectedfrom an ether-based solvent, an ester-based solvent, an amide-basedsolvent, or a mixture of two or more thereof.
 9. The lithium secondarybattery of claim 8, wherein the ester-based solvent comprises at leastone compound selected from a cyclic carbonate compound, a linearcarbonate compound, a linear ester compound, or a cyclic ester compound.10. The lithium secondary battery of claim 1, wherein, in Formula 1, R₁is an alkylene group having 2 carbon atoms which is substituted with onefluorine atom, or an alkylene group having 2 carbon atoms which issubstituted with an alkyl group having 1 to 3 carbon atoms that includesat least one fluorine atom.
 11. The lithium secondary battery of claim1, wherein the first additive comprises at least one selected fromcompounds represented by Formulae 1b to 1d, 1f or 1h:


12. The lithium secondary battery of claim 1, wherein the first additiveis included in an amount of 0.1 wt % to 10 wt % based on a total weightof the non-aqueous electrolyte solution.
 13. The lithium secondarybattery of claim 1, further comprising a second additive selected fromvinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate,cyclic sulfite, saturated sultone, unsaturated sultone, a non-cyclicsulfone, an alkylsilyl compound, an inorganic additive, or a combinationthereof.
 14. The lithium secondary battery of claim 13, wherein thesecond additive is included in an amount of about 0.01 wt % or morebased on a total amount of the non-aqueous electrolyte solution.